X-ray security inspection systems for the inspection of cargo and shipping containers typically use transmission radiographic techniques. FIG. 1A depicts a cargo inspection system employing such a technique. A fan-shaped beam 12 of penetrating radiation, emitted by a source 14, is detected by elements of a detector array 16 distal to a target object, here truck 10, is used to produce images of the target object. The thickness of material to be penetrated by the X-rays may exceed 300 mm of steel equivalent in some cases. To insure the required penetration, inspection systems typically use X-rays with a maximum energy of several MeV, currently up to about 9 MeV. X-rays in excess of 1 MeV are frequently referred to as hard X-rays or high-energy X-rays. While the invention described herein pertains to any penetrating radiation, it may be described, purely as a matter of heuristic convenience, in terms of high-energy X-rays.
Information (such as mass absorption coefficient, effective atomic number Zeff, electron density, etc.) with respect to the material composition of the contents of objects may be obtained on the basis of the interaction of X-rays with the material, and, more particularly, by illuminating the material with X-ray beams having energy spectra with more than one distinct energy endpoint (peak energy), or by employing energy discriminating detectors. Dual energy methods of material discrimination are widely used in X-ray inspection systems for security control of hand luggage in customs and other security checkpoints. Dual energy inspection is discussed in the following references, for example, which are incorporated herein by reference:    U.S. Pat. No. 5,524,133, Neale et al., “Material Identification using X-Rays” (1996) (hereinafter, “Neale '133”)    U.S. Pat. No. 7,257,188, Bjorkholm, “Dual Energy Scanning of Contents of an Object” (2005)    U.S. Pat. No. 6,069,936, Bjorkholm, “Material Discrimination using Single-Energy X-Ray Imaging System” (2000)
More recently, the dual energy methods have been extended to high-energy inspection systems for cargo containers, where they are less effective due to the weaker Z-dependence of the dominant interaction.
In the practice of dual-energy inspection, X-ray transmission data of an inspected object are obtained for both energies, and processed by computer, whereupon a resulting image is displayed on a monitor, typically in a special color palette that facilitates visual identification of contraband or hazardous materials. More particularly, special computer software may identify various materials and artificial colors may be assigned to various values of Zeff.
A typical energy range for the inspection of smaller objects is below 0.5 MeV, taking advantage of the strong Z-dependence of the X-ray attenuation coefficient due to the prevalence of the photoelectric interaction (characterized by a cross-section, ˜Z4-Z5) at lower energies. In the range of 1-10 MeV, however, X-ray interaction is dominated by the Compton effect with its weak dependence of attenuation coefficient (mass absorption) on the atomic number: μc˜Z/A (which is approximately constant and equal to 0.5), where Z denotes atomic number, and A denotes atomic mass, which is to say that the mass absorption coefficient is largely Z-insensitive in the energy regime dominated by Compton scatter. The relative importance of the three major X-ray interactions for different Z-values at energies between 10 keV and 100 MeV is shown in FIG. 1B.
Expanding upon the principles of dual-energy materials discrimination, composition analysis and explosives detection using triple energy X-ray transmission were the subject of a 1993 Department of Transportation SBIR grant to Advanced Optical Technologies, while application of triple energy in the context of X-ray computed tomography was studied by Dukovic et al., in “Basis material decomposition using triple-energy reconstructions for X-ray tomography,” IEEE Instr. and Meas. Technology Conf., Venice, vol. 3, pp. 1481-83 (1999).
As an example of dual-energy materials discrimination, Neale '133 discusses scanning systems for large objects such as freight in a container or on a vehicle. In the system depicted in FIG. 14 of Neale '133, two stationary sources of X-ray radiation are provided, each source emitting a beam that is collimated into a fan beam. The sources face adjacent sides of the freight and the fan beams are perpendicular to each other. A stationary detector array is located opposite each source, on opposite sides of the freight, to receive radiation transmitted through the freight. In addition, X-ray radiations of two different energies are emitted by each source. One energy is significantly higher than the other. For example, energies of 1 MeV and 5 or 6 MeV may be used. A ratio of the mean number of X-rays detected at each energy endpoint by the detector array as a whole for each slice or by the individual detectors of the array is determined and compared to a look up table to identify a mean atomic number corresponding to the ratio. The material content of the freight is thereby determined.
Tandem-detector configurations, in which a rearward detector is used to detect higher-energy photons that have traversed a forward low-energy detector, may be used for dual-energy inspection at low X-ray energies. However, tandem-detector configurations tend to be ineffectual for inspection at energies above ˜1 MeV, because the beam is typically so hardened by traversal of the intervening cargo that there is little differential detection between the detector elements. Moreover, the signals from each element of a tandem detector are also typically cross-contaminated by Compton scattered photons: the forward low-energy detector signal is contaminated by backscattered photons, whereas the signal produced in the rearward high-energy element is contaminated by forward scattered photons.
The use of dual energy beams, however, gives rise to ambiguity in determining the atomic number of a sample. In particular, the teachings of Ishkhanov, et al., “Multi-beam methods of atomic number discrimination,” Preprint, SINP, Moscow, (2005) (in Russian), and Ishkhanov, et al., “Multiple-Beam Method for Object Scanning,” Bulletin of the Russian Academy of Science: Physics, 2008, Vol. 72, No. 6, 859-62, (2008) present the probability distribution, as shown in FIG. 8, infra, of determining the effective atomic number for a uranium (Z=92) object 4.5×4.5×4.5 cm3 in size, as simulated for double end-point energy (thin line) and triple end-point energy (bold line) methods. It is evident that a substantial probability is attributed to an incorrect range in the vicinity of Z˜68.
A further disadvantage of currently practiced multiple-energy techniques involves the use of linear accelerators (linacs) to generate X-ray pulses in the MeV range used for cargo inspection. Linac pulses are typically separated by two milliseconds, or more, during the course of which interval the position of the beam has moved relative to the cargo. It would be preferable, however, to ensure that all energies used in the analysis of cargo characteristics sample exactly the same part of the cargo. It would be desirable, therefore, to provide a method for applying multiple energy techniques in the 4-10 MeV range for recognition of groups of materials according to their effective atomic number without recourse to multiple beams, separated in space or time.
In cargo inspection applications, the wide range of densities in the inspected volume may cause the X-ray attenuation, on traversal of the cargo, to vary by as much as a factor of 100,000. This variation requires an equivalent dynamic range for the detection system, a daunting challenge to effective inspection techniques.